1. Field of the Invention
The present invention relates generally to control systems for vehicle engines coupled with automatic transmissions, and more particularly, to a system for controlling an output of a vehicle engine which is coupled with an automatic transmission employed in a vehicle so as to suppress torque shocks brought about by speed change operations performed in the automatic transmission.
2. Description of the Prior Art
There has been commonly used in vehicles an automatic transmission which is coupled with an engine provided in the vehicle and includes a torque converter comprising a pump impeller, a turbine runner and a stator, and a power transmitting gear arrangement including a speed change mechanism connected to the turbine runner in the torque converter. Such an automatic transmission employed to be coupled with the engine in the vehicle is usually accompanied with a hydraulic pressure control device including an oil hydraulic control circuit, by which engaging friction elements, such as clutch and brake elements, operating with oil pressure in the speed change mechanism are caused to work for a speed change operation.
When the speed change operation is carried out in the automatic transmission accompanied with the hydraulic pressure control device, sudden rise or down in speed of the engine is caused in response to variations in gear ratio in the speed change mechanism and thereby sudden variations in torque are caused on an output shaft of the automatic transmission. This results in that torque shock on the vehicle is brought about by each speed change operation performed in the automatic transmission.
With the intention of suppressing the torque shock resulting from the speed change operation, it is considered to adjust oil pressure applied to the engaging friction elements so that each engaging friction element comes into engagement or disengagement gradually and smoothly. In such a case, however, a period of time in which the engaging friction element is kept in a slipping condition in process of the speed change operation is protracted and therefore it is feared that the engaging friction element is subjected to seizure and abrasion of great degree.
In view of this, there has been proposed to reduce torque produced by the engine (engine torque) for a predetermined duration on the occasion of the speed change operation so as to suppress the torque shock resulting from the speed change operation, as disclosed in, for example, the Japanese patent application published before examination under publication number 61-104128. In the case where the engine torque is thus reduced for suppressing the torque shocks, for example, ignition timing is selected as a controlled subject for varying the engine torque and controlled to be retarded, compared with a normal ignition timing, so as to reduce the engine torque during the speed change operation.
In the control for varying the engine torque so as to suppress the torque shock resulting from the speed change operation as described above, it is required to detect a starting time point at which the speed change operation is actually commenced in the automatic transmission and an ending time point at which the speed change operation is actually terminated in the automatic transmission. For fulfilling such a requirement, since it is quite difficult to detect directly the starting and ending time points on the strength of variations in the operating condition of each of the engaging friction elements employed in the speed changing mechanism in the automatic transmission, each of the starting and ending time points is determined by means of calculation. The starting time point at which either a shifting-up operation or a shifting-down operation is to be commenced can be calculated relatively easily and accurately in consideration of delay in operating oil supply to the engaging friction elements in the speed change mechanism, variations in engine speed and so on. The ending time point at which the shifting-up operation is to be terminated can be also calculated relatively easily and accurately based on an engine speed which is to be taken immediately after the shifting-up operation and calculated based on an engine speed taken immediately before the shifting-up operation and gear ratios taken in the speed changing mechanism before and after the shifting-up operation, respectively, because the shifting-up operation is usually carried out without bringing about variations in output of the engine. To the contrary, the ending time point at which the shifting-down operation is to be terminated is not able to be accurately obtained be means of calculation conducted in the same manner as the calculation of the ending time point at which the shifting-up operation is to be terminated because the shifting-down operation is usually carried out with increase in gear ratio in the speed change mechanism and increase in output of the engine.
However, in the prior art, the above mentioned difficulty in calculation of the ending time point of the shifting-down operation has not been taken into consideration, and both the ending time point of the shifting-up operation and the ending time point of the shifting-down operation are calculated based on the engine speed in the same manner. Therefore, it is feared that a calculated ending time point of the shifting-down operation does not coincide with an actual ending time point of the shifting-down operation, so that the control for varying the engine torque so as to suppress the torque shock resulting from the speed change operation carried out in the automatic transmission is not performed timely.